Infection by pathogenic bacteria requires the attachment of microbes to host cell surfaces. Among the factors required for this process are type IV pili. Pili are adhesive cell surface organelles that mediate the attachment of virtually all pathogenic Gram negative bacteria to their eukaryotic hosts. Pili also mediate twitching motility, one mechanism of bacterial motion. Type IV pili are composed of a single structural subunit, pilin, and possibly other associated proteins. To more fully understand type IV pilin subunit structure and chemistry we are studying a second pilin, Pseudomonas aeruginosa. N. gonorrhea and P. aeruginosa pilin share a sequence identity of 32%.